1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method.
2. Related Background Art
An image scanner using a CCD linear image sensor (hereafter referred to as CCD) has been known so far as an image reading apparatus.
FIGS. 32A and 32B briefly show the structure of a flat-bed-type scanner, in which FIG. 32A is a top view and FIG. 32B is a side view. Reference D denotes an original to be read put on an original table glass 100. The original is irradiated with a light source 101 and the reflected light is turned back by mirrors 102, 103, and 104 to focus the original on a CCD 106 by a lens 105. The original D is entirely read by moving a reading unit 107 with the light source 101, mirrors 102, 103, and 104, lens 105, and CCD 106 firmly mounted on it from the left to right in FIG. 32A in parallel with the original table glass 100 and thereby scanning the original D to obtain image signals for one page from the CCD 106. In this case, as shown in FIG. 32A, the direction from the top toward the bottom denotes a main scanning direction and the direction from right to left denotes a sub-scanning direction.
FIGS. 33A and 33B briefly show the structure of a CCD. In FIG. 33A, references 201-a, b, c, d . . . denote a photoelectric-conversion pixel string of 202-a, b, c, d . . . denote carrying gates for carrying electric charges photoelectric-converted by the photoelectric-conversion pixel string of 201-a, b, c, d, . . . , 203 denotes a transfer section for successively transferring the carried electric charges, and 204 denotes an output circuit for linearly reading the transferred electric charges as output signals. FIG. 33B shows the photoelectric-conversion pixel portions 201-a and 201-b in FIG. 33A by enlarging them.
As described above, the original D is linearly illuminated in the main-scanning direction and an optical image focused on the photoelectric-conversion pixel string of 201-a, b, c, d, . . . moves in the sub-scanning direction shown in FIG. 33B at a predetermined speed through an optical lens or the like. Electric charges photoelectric-converted by the photoelectric-conversion pixel string of 201-a, b, c, d, . . . and accumulated are carried to a transfer section 20 in a predetermined period when the focused image moves from the position A to position B shown in FIG. 33B and then, the focused image is read from the output circuit 204 in a predetermined period when the image moves from the position B to the position C. Hereafter, the above operations are repeated and thereby, a cyclic line sequential signal, that is, a main-scanning line signal is obtained.
Generally, as shown in FIG. 33B, a distance AB (and a distance BC) is set equally to a photoelectric-pixel pitch P in the main-scanning direction so that the same resolution can be obtained in main scanning direction and sub-scanning direction.
In the case of the above conventional example, however, to raise the resolution up to two times, it is necessary to decrease the photoelectric-conversion pixel pitch P to xc2xd. Therefore, the pixel size of the photoelectric-conversion pixel string of 201-a, b, c, d, . . . must be decreased to xc2xd in main scanning direction and sub-scanning direction and the period for the above reading must be also reduced to xc2xd. Therefore, the area of photoelectric conversion pixels is decreased to xc2xc and moreover, the time for photoelectric conversion is decreased to xc2xd. Thus, problems occur that the sensitivity is lowered to xe2x85x9 and the image quality is greatly deteriorated.
Moreover, because the transfer rate is doubled, the electric-charge transfer performance is deteriorated and a problem occurs that the heat produced in the transfer section and the power consumption are increased. It is needless to say that these factors deteriorate the image quality of an image reading apparatus. Moreover, because the pixel size decreases, it is necessary to improve the resolution of a lens and thus, the lens cost is increased.
Among the above problems, for the problem of insufficient sensitivity, a method of increasing the size of a photoelectric-conversion pixel and decreasing the focusing magnification of a lens is considered. For example, by increasing the size of photoelectric-conversion pixel up to substantial 2.8 times in main scanning direction and sub-scanning direction, the sensitivity becomes equal to the conventional sensitivity. However, because the size of the transfer section also increases, deterioration of the transfer performance is further progressed and the produced heat is further increased and thus, the image quality is further deteriorated.
Moreover, the chip size in the main-scanning direction is increased up to substantial 2.8 times and thereby, a problem occurs that costs of a CCD and a lens are greatly increased.
It is an object of the present invention to provide a an image reading apparatus capable of realizing a high image quality equal to two-fold high resolution without changing CCDs or lenses or deteriorating characteristics.
To solve the above problems, according to one aspect of the present invention, there is provided an image reading apparatus comprising optical means for focusing the reflected optical image of an original, an image sensor constituted by linearly arranging pluralities of photoelectric-conversion pixels to accumulate linear reflected optical images focused by the optical means in the photoelectric-conversion pixels as signal electric charges and successively output the linear reflected optical images as image signals every certain period, scanning means for scanning the original in a sub-scanning direction vertical to a main scanning direction serving as the direction of the linear reflected optical images, and pixel-shifting means for shifting the relative positions between the position of an optical image focused on the image sensor through the optical means and the photoelectric-conversion pixels on the unit basis of substantial 1/N (N is an integer) the pixel pitch of the image sensor in the main scanning direction.
Moreover, according to an another aspect of the present invention, there is disclosed an image reading method comprising steps of picking up reflected optical images of an original focused by optical means with a linear image sensor constituted by linearly arranging a plurality of photoelectric-conversion pixels and thereby, successively outputting image signals corresponding to the linear reflected optical images every certain period, shifting the relative positions between the position of an optical image focused on the linear image sensor through the optical means on one hand and the photoelectric-conversion pixels in a main scanning direction on the unit basis of substantial 1/N (N is an integer) the pixel pitch of the linear image sensor every plurality of repetitions of sub-scanning when the original is scanned in a sub-scanning direction vertical to a main scanning direction of the linear reflected optical images, and thereby picking up the original.
Furthermore, according to an another aspect of the present invention, there is disclosed a storage medium storing an image-reading program comprising steps of picking up reflected optical images of an original focused by optical means with a linear image sensor constituted by linearly arranging plurality of photoelectric-conversion pixels to successively output image signals corresponding to the linear reflected optical images every certain period, scanning the original a plurality of times in a sub-scanning direction vertical to a main scanning direction of the linear reflected optical images, shifting the relative positions between the position of an optical image focused on the linear image sensor through the optical means and the photoelectric-conversion pixels in a main scanning direction on the unit basis of substantial 1/N (N is an integer) the pixel pitch of the linear image sensor at every plurality of repetitions of sub-scanning, and synthesizing a plurality of picked-up images.
Furthermore, it is another object of the present invention to provide an image reading apparatus capable of obtaining a high resolution equal to that of an image sensor substantially having the number of pixels two times or more without increasing the number of pixels of the image sensor and preventing the MTF (Modulation Transfer Function) from deteriorating.
To achieve the above object, according to an aspect of the present invention, there is provided an image reading apparatus comprising an image sensor constituted by linearly arranging a plurality of photoelectric-conversion pixels, scanning means for scanning an original in a sub-scanning direction vertical to a main scanning direction serving as the direction of linear reflected optical images incoming into the image sensor, pixel-shifting means for shifting the relative positions between the position of an optical image focused on the image sensor and the photoelectric-conversion pixels by substantial xc2xd the pixel pitch of the image sensor in a main scanning direction, control means for controlling the scanning means to perform scanning operation two times at a pitch substantially equal to the pixel pitch of the image sensor at respective positions sifted to each other in a sub-scanning direction by a value substantially corresponding to xc2xd the pixel pitch of the image sensor and controlling the image-shifting means to cause the means to perform pixel shifting of xc2xd the pixel pitch between first-time sub-scanning and second-time sub-scanning, and image-signal-generating means for generating an image signal by performing the averaging operation between diagonal adjacent pixels for the pixel data for each pixel obtained through the above two repetitions of sub-scanning.
Other objects and features of the present invention will become more apparent from the specification and drawings shown below.